In this presentation about shape optimization by the level set method, p.2, the following definition is given:
Find the most rigid structure $\omega$, of prescribed volume, contained into a given domain $\Omega$, when given external forces are applied.
$$\inf_{\omega \subset \Omega, |\omega|=V} J(\omega)$$
In the above definition, what's the meaning of $$|\omega|=V$$?